Within the meaning of this application, an EUV lithography system is understood to be an optical system for extreme-ultraviolet (EUV) lithography, that is to say an optical system which can be used in the field of EUV lithography. Alongside an EUV lithography apparatus which serves for producing semiconductor components, the optical system can be, for example, an inspection system for inspecting a photomask (also called reticle hereinafter) used in an EUV lithography apparatus, for inspecting a semiconductor substrate (also called wafer hereinafter) to be patterned, or a metrology system used for measuring an EUV lithography apparatus or parts thereof, for example for measuring a projection system. In such an EUV lithography system, typically reflective optical elements, for example reflective multilayer mirrors, are arranged in a vacuum environment in one or a plurality of housings or vacuum chambers.
WO 2004/104707 A2 has disclosed an apparatus and a method for cleaning at least one optical component wherein a reaction partner, which is substantially transparent to the radiation of an EUV radiation source, depending on the prevailing reaction conditions, is admitted into a vacuum chamber via a feed device in order to chemically react with contaminating deposits for the purpose of eliminating the latter from the optical component. The reaction partner can be a hydrogen-containing substance that forms a volatile reaction product with the deposits under the prevailing reaction conditions. With the exception of the optical components, the remaining constituents in a vacuum chamber can be chemically passivated by the local addition of additional reaction gases, e.g. oxygen or hydrogen, or volatile substances can be converted into non-volatile substances. The reaction partner e.g. in the form of molecular hydrogen can be converted into radicals by the radiation of the EUV radiation source or by irradiation with an additional excitation device in order to intensify the cleaning effect.
In an EUV lithography system, hydrogen can be used in molecular form also as a purge gas or as an inert gas. In this regard, for example, WO 2008/034582 A2 in the name of the applicant discloses performing a local encapsulation of components at risk of contamination, in particular optical surfaces, of an EUV lithography system into partial housings having delimited partial volumes (mini-environments), which are purged with a purge gas, e.g. with hydrogen, in order to make it more difficult for contaminating substances to enter from the surroundings of the partial housing. Particles released within the mini-environment are intended to be entrained with the purge gas flow and transported into the surroundings via an outlet. However, the hydrogen used as a purge gas is also exposed to the EUV radiation and can be converted into radicals and in particular ionized if the EUV radiation has a sufficient energy.
Ionized hydrogen together with further ionized species and with electrons forms a plasma in the residual gas atmosphere of the EUV lithography system, which is referred to as hydrogen plasma hereinafter. The plasma density and the energy of the hydrogen ions or of the hydrogen radicals increase as the power of the EUV radiation increases. Alongside the positive effects discussed further above as a result of the cleaning action of the hydrogen plasma with regard to carbon or hydrocarbon contaminations, the hydrogen plasma generated by the EUV radiation also has negative effects.
A hydrogen plasma reacts for example upon contact with components which are arranged in the vacuum environment and which comprise specific critical materials, in particular metals such as tin or zinc, to form readily volatile hydrogen or hydride compounds or hydride complexes (hereinafter: hydrides) that are outgassed into the vacuum environment. The outgassed hydrides can deposit as contaminating substances on the optical surfaces of optical elements and react with the materials present there to form chemical compounds of low volatility such as e.g. metal alloys. The formation of the hydrides is a chemical reaction below a threshold value at which physical erosion (as a result of sputtering) commences.
The contaminating substances outgassed on account of the hydrogen plasma and deposited on the optical surfaces are generally far more difficult to remove from the optical surfaces than other contaminants that typically occur, e.g. in the form of hydrocarbon compounds. The optical elements typically at least partly lose their functionality on account of the deposits in the form of hydrides. In particular, the reflectivity of the optical elements can decrease, as a result of which the throughput of the EUV lithography system, in particular of an EUV lithography apparatus, decreases.